US 2004/0104044 A1 discloses a printed circuit board design. To electrically interconnect two transmission lines situated in different layers of the printed circuit board, copper foils imprinted on the sidewalls of the printed circuit board are used. In case the transmission lines have equal line widths, the copper foil has a rectangular shape. In case the transmission lines do not have equal line widths, one end of the copper foil connected to a broadest transmission line has a foil width equal to a line width of this broadest transmission line, and the other end of the copper foil connected to a narrowest transmission line has a foil width equal to a line width of this narrowest transmission line.
To electrically interconnect different transmission lines having different line widths and situated in a same layer, a so-called taper is to be used. This taper interconnects the transmission lines, preferably such that a largest part of radio frequency signals can pass the taper and only a smallest part of these radio frequency signals is reflected. A reflection coefficient of the taper defines which part is reflected. The reflection coefficient for example depends on the fact that the transmission lines, when having different line widths, will have different impedances. To keep this reflection coefficient as small as possible, one end of the taper connected to a broadest transmission line may have a taper width equal to a line width of this broadest transmission line, and the other end of the taper connected to a narrowest transmission line may have a taper width equal to a line width of this narrowest transmission line. From the one end to the other end of the taper, the taper width may gradually change, for example linearly or exponentially.
The reflection coefficient further depends on a distance between ends of the transmission lines. As long as this distance is larger than 20% of a wavelength of the frequency signals to be exchanged via the transmission lines, the reflection coefficient is relatively small. But for shorter distances, this reflection coefficient becomes relatively large.